1. Field of the Invention
This invention relates to a position signal producing apparatus, for use with an exposure apparatus, for producing a position signal indicative of position along an axis of a wafer on which an image of a reticle is to be projected.
2. Description of the Prior Art
Recently, in semiconductor manufacturing apparatus, a demand for high density arrangement of elements has been increasingly developed, so that thickness of circuit patterns of each element are reaching to a value lower than 0.5 .mu.m. In exposing processes of such fine circuit patterns, it is extremely important to consist one pattern position with another pattern at every exposing process to carry out many times of overwrite exposure which is necessary for producing semiconductor products. Such processing requires positioning accuracy of less than 0.1 .mu.m.
A position signal producing apparatus of a prior art is disclosed in Japanese Patent Laid-open Publication No. 63-78004. Hereinbelow will be described the above-mentioned prior art position signal producing apparatus with reference to FIG. 13.
FIG. 13 diagrammatically shows the arrangement of a prior art exposure apparatus in which the position signal producing apparatus is incorporated. In FIG. 13, alignment light emitted from a laser light source 101, which is coherent and includes two frequency components, is split into two light beams by a half mirror 102 and is diffracted by a first pair of diffraction gratings 104a and 104b formed on a reticle 103, respectively. Each diffracted light is projected onto a second pair of diffraction gratings 110a and 110b provided on a wafer 109 through a first lens system 105a and 105b, a spatial filter 106a and 106b, a second lens system 107a and 107b, and a project lens system 108. The diffracted light 111a and 111b is introduced into photodetectors 112a and 112b in the opposite direction through the project lens system 108 and the second lens systems 107a and 107b. When two beams are projected onto the second diffraction gratings 110a and 110b on the wafer 109 with suitable directions, respectively, each diffraction light is diffracted in such a direction that diffracted light overlaps each other and interfere with each other. Light intensities of a pair of interfered diffracted light are detected by the photodetectors 112a and 112b. The detected results are compared by a comparator 113. In accordance with the detected results, a control system 114 drives the wafer 109, so that the difference of the above-mentioned diffracted light intensity equals zero. Thus, position alignment between the reticle 103 and the wafer 109 is performed.
On the other hand, the pattern of the reticle 103 is illuminated by a projecting light source 115 and an illumination optical system 116. The project image is focused onto the wafer 109 through the project lens system 108.
However, the above-mentioned prior art arrangement is effective only when the wavelength of exposure light emitted from the projecting light source 115 and the wavelength of alignment light emitted from the laser light source 101 are substantially equal to each other and the project lens system 108 displays good focusing characteristic to both light. It is very difficult to build up an achromatic project lens system where ultraviolet light is used for the exposure light, on the other hand visible light is used for the alignment light. This is because kinds of glass materials for making up a dioptric system are limited at wavelength of ultraviolet light. For example, ultraviolet light is generated by a KrF excimer laser.
In other words, the project lens system 108 is so designed as to sufficiently achromatize at exposure wavelength and thus, shows large chromatic aberration for other wavelength light. Therefore, it is desired that wavelength of the alignment light is sufficiently close to that of the exposure light. However, from a point of view of producing semiconductor products, it is desired that the wavelength of the alignment light is sufficiently separated from that of the exposure light. This is because a high sensitive resist such as a chemically sensitized resist may be exposed by and respond to the alignment light with a high sensitivity. Further, a multilayer resist used for compensation of limit of focusing characteristic of the project lens system 108, or a resist layer containing die for preventing multireflection may absorb the alignment light whose wavelength is close to that of the exposure light. In other words, the wavelength of the alignment light should be sufficiently separated from that of the exposure light from the reasons involved in the semiconductor manufacturing process. Therefore, in the above-mentioned prior art arrangement, there is a problem that position alignment between the reticle and the wafer with high accuracy is difficult to perform.